The inner ear is a sensory organ of which the function is to transcribe vibration waves between 0 and 20 kHz from the environment into a sensory inflow. It includes two physically distinct portions having two different respective functions, namely the balance portion and the hearing portion.
The balance portion, called the vestibule, has the function of encoding vibration waves between 0 and 800 Hz. It also makes it possible to situate the direction of the head with respect to its environment, by acceleration measurement systems in the semicircular canals. The hearing portion, called the cochlea, encodes the vibration waves between 20 Hz and 20 kHz.
The pathologies that may affect the inner ear can primarily be classified into three categories, namely loss of auditory acuity, balance disorders and tinnitus.
The main cause of these pathologies is associated with the natural aging of the sensory cells, resulting in hearing disorders (presbycusis, age-related deafness) and balance disorders (vestibulopathy, age-related hypoflexia).
Some diseases, such as Meniere's disease, can trigger all three types of pathologies (deafness, tinnitus and vertigo).
There are a number of possibilities for treatment of these pathologies. For some cases of deafness associated with transmission and vestibular disorders, there are surgical treatments, such as the opening of endolymphatic sacs or vestibular nerve section or middle ear surgeries. These treatments have the disadvantage of being irreversible.
Some drugs or rehabilitation treatments can attenuate vertigo, tinnitus and sudden or fluctuating deafness. These treatments do not cure the disease, but simply enable the affects thereof to be reduced. These pathologies can also be treated with an apparatus. However, there is currently no apparatus enabling all of these pathologies to be treated.
For deafness, hearing aids are designed to amplify the acoustic waves. They are commonly used to treat all levels of deafness, from slight to profound. There are also systems implanted into the middle ear, which are designed to mechanically amplify the movements of the ear ossicles (U.S. Pat. Nos. 5,913,815 and 6,293,903). There are systems for bone conduction, by means of a vibrator integrated in a pair of eyeglasses, for example, which are designed to cause the skull to vibrate so as to transmit the vibration wave to the inner ear. These latter systems are limited to the treatment of deafness associated with a transmission problem, because the pressure exerted on the skin to transmit the vibration to the skull must be limited. There are also implanted bone conduction systems in which the vibrator is either implanted or can be connected by an opening in the skin to an implant attached to the skull (U.S. Pat. No. 4,498,461, WO 02/09622). Electric stimulators (intra- or extra-cochlear implants), directly stimulating the auditory nerve, are used in cases of significant deafness, i.e. when the acoustic waves are insufficient for stimulating the auditory nerve.
There is no apparatus for vertigo. For tinnitus, a masking system has been proposed, which sends a noise to the inner ear to mask the tinnitus (U.S. Pat. No. 5,325,872). In addition, external hearing aids make it possible to correct tinnitus associated with deafness: the correction of the deafness leads to a reduction in the tinnitus. Electric stimulators of the inner ear are also used to reduce tinnitus, when the deafness is too severe to be treated acoustically.
Hearing aids and masks have the advantages of not requiring surgery and of being reversible and compatible with MRI. However, they are generally relatively visible, and therefore unaesthetic. In addition, they sometimes have contraindications (aplasia of the outer ear, external otitis, eczema, and so on). They often cause acoustic feedback due to their structure and in particular the closeness of the microphone to the earphone. Some of these apparatuses require plugging the auditory canal, which raises the problem of amplification of low-frequency sounds, and is often disturbing for the patient who then hears more body sounds (chewing, blood circulation). Moreover, these apparatuses operate only on a frequency band between 125 and 6000 Hz due to the use of an earphone.
Systems implanted in the middle ear, on the other hand, are discreet and do not require blocking of the auditory canal. They cause little acoustic feedback, thereby allowing for more high-frequency sounds than with hearing aids. They cause less distortion and operate in a wider frequency band (up to 10 kHz) because they do not use an earphone. However, they require surgery of the middle ear and general anesthesia, with all of the risks of these operations (facial nerve, ear ossicles), which makes them relatively irreversible and incompatible with MRI and radiotherapy, and raises problems in the event of a breakdown or failure of the apparatus. By comparison with hearing aids, these apparatuses are relatively expensive, and if the deafness changes, their capacity for adjustment is limited due to the use of an electromagnetic transducer. Moreover, their bandwidth does not cover the entire spectrum to which the inner ear is normally sensitive (limited to frequencies between 125 Hz and 10 kHz).
Non-implanted or semi-implanted bone conduction systems have the advantage of providing quality sound. They do not require the auditory canal to be plugged either, and do not generate acoustic feedback. However, they are highly visible and therefore unaesthetic, and consume a large amount of power. Moreover, they provide little or no left/right selectivity due to the mode of transmission used (through a skull bone). Non-implanted systems must clamp the skin (conduction of vibrations through the skin), which is uncomfortable for the patient, and even painful, and can cause necrosis of the skin. Semi-implanted systems requiring a permanent opening in the skin barrier lead to risks of infection.
Cochlear implants require complex surgery, therefore with risks, which is irreversible, presenting a problem in the event of failure of the treatment or breakdown of the apparatus. They are incompatible with imaging systems, expensive, and their bandwidth is limited to frequencies between 125 and 6000 Hz.